The present invention relates to a method for preventing or inhibiting deactivation of fresh Cs-promoted catalysts. The invention also relates to a method for restoring lost catalytic activity and selectivity of fresh cesium-promoted catalysts, particularly, fresh cesium-promoted, silver catalysts. The invention further relates to a method for preparing 3,4-epoxy-1-butene using the restored cesium-promoted, silver catalysts.
Cesium (Cs) salt-promoted catalysts are typically used for the selective epoxidation of olefins to their corresponding olefin epoxides. For example, CsCl, CsOH, and/or Cs2O salts are used as silver (Ag) promoters for the selective epoxidation of ethylene to ethylene oxide; and CsCl is used as a Ag promoter for the selective epoxidation of butadiene to form 3,4-epoxy-1-butene.
Cs salts and Cs salt-containing compositions are known to be very hygroscopic and moisture-sensitive. Moisture-sensitivity can result in physical and chemical changes in Cs-containing compositions. Thus, Cs salt-promoted, Ag catalysts, for example, which are very active and selective for the epoxidation of olefins to their olefin epoxides, can lose their activity and selectivity when the promoter interaction of the Cs salt or Cs component with the Ag catalyst is lost or modified by the moisture-sensitivity of the Cs component.
It has been found that fresh Cs-promoted catalysts undergo loss of activity and selectivity for the selective epoxidation of olefins to their corresponding olefin epoxides upon storage under ambient conditions due to exposure to ambient moisture. It has been discovered that this loss in catalytic activity and selectivity can be prevented or inhibited by storing the fresh catalyst in a substantially moisture-free environment. It has also been surprisingly discovered that, if the catalyst has been exposed to moisture, and there is a loss of catalytic activity and selectivity, the initial catalytic activity and selectivity can be essentially completely restored by heating (calcining) the catalyst in the presence of a sweep gas at conditions effective to remove the moisture and restore the Cs-Ag promoter interaction.
Moisture-sensitivity and loss of activity/selectivity are related to the level of Cs salt promotion. Thus, catalysts promoted with higher levels of Cs salt deactivate upon storage more quickly than catalysts promoted with lower levels of Cs salt. However, all fresh Cs salt-promoted catalysts, regardless of the level of promoter loading, are moisture-sensitive and are prone to deactivation upon storage. Thus, all such catalysts would benefit from storage in a substantially moisture-free environment and/or from a calcination treatment to maintain and/or give optimum catalytic activity and selectivity.
Accordingly, in accordance with one aspect of the present invention, there is provided a method for preventing or inhibiting deactivation of fresh Cs-promoted catalysts. The method comprises the step of maintaining the fresh Cs-promoted catalyst in a substantially moisture-free environment until it is used.
In accordance with another aspect of the present invention, there is provided a method for restoring lost activity and selectivity of fresh Cs-promoted catalysts. The method comprises heating the catalyst in the presence of a sweep gas at conditions effective to restore the lost activity and selectivity immediately prior to using the catalyst.
In accordance with yet another aspect of the present invention, there is provided a method for preparing 3,4-epoxy-1-butene. The method comprises the steps of heating a fresh cesium-promoted, silver catalyst in the presence of a sweep gas at conditions effective to restore lost activity and selectivity of the catalyst; and, immediately thereafter, contacting 1,3-butadiene With oxygen in the presence of the catalyst at conditions effective to form 3,4-epoxy-1-butene.
As used herein, xe2x80x9cfreshxe2x80x9d catalyst means catalyst that has undergone all of the preparation steps including calcination to thermally reduce the valence state of the promoted metal and that is otherwise ready for use, except that it has not been contacted with reactive gases such as ethylene or butadiene under reaction conditions.
Also, as used herein, xe2x80x9cimmediately prior toxe2x80x9d and xe2x80x9cimmediately thereafterxe2x80x9d mean that the heat-treated catalyst has not been exposed to moisture for a period of time such that the restorative effect of the heat-treating step is lost.
Further, as used herein, xe2x80x9csubstantially moisture-free environmentxe2x80x9d means an environment that contains moisture at levels below that of ambient conditions.
Without wishing to be bound by theory, it is believed that the moisture-sensitivity of the Cs salt promoter results in the migration and agglomeration of the promoter, either into xe2x80x9cpuddlesxe2x80x9d on the Ag surface, or from the Ag surface onto the support material such as A12O3. Consequently, the promoter undergoes a transition from being optimally distributed in a two-dimensional array on the Ag surface to a situation where much of the Ag surface is not promoted. The result is lower activity and selectivity. Deactivation begins to occur for Cs-promoted, Ag catalysts as soon as they are prepared.
Various Cs-promoted, Ag catalysts as well as their methods of preparation are well known in the art. Representative Cs salt promoters include cesium nitrate, cesium chloride, cesium bromide, cesium oxide, cesium hydroxide, cesium acetate, cesium sulfate, cesium perrhenate (Cs2Re2O7), and the like. Representative silver compounds include silver nitrate, silver oxalate, silver acetate, and the like. Such catalysts and their methods of preparation are described in the patent literature such as WO 89/07101 and U.S. Pat. Nos. 4,555,501; 4,950,773; 5,155,242; 5,362,890; and 5,691,269; the entire contents of which are hereby incorporated by reference.
The rate of deactivation of Cs-promoted, Ag catalysts is strongly ( greater than 1st order) and directly proportional to the level of Cs salt promotion. Thus, the moisture-sensitivity of the Cs salt promoter is an intrinsic property of all Cs salt promoted, Ag catalysts.
I have found that this moisture-induced deactivation can be prevented or inhibited by storing the fresh Cs-promoted catalyst in a substantially moisture-free environment until it is ready for use. As will be readily apparent to those skilled in the art, this moisture-free environment may be provided in various ways. For example, the fresh Cs-promoted catalyst may be stored in a vacuum desiccator or in a closed container containing desiccators. Preferably, the moisture-free environment has a moisture content of 1000 ppm H2O vapor or less.
If, however, the Cs-promoted catalyst has been exposed to moisture and has lost some of its initial activity/selectivity, I have surprisingly found that the Cs salt promoter can be very efficiently re-dispersed back on the Ag surface by simple calcination treatments in the presence of a sweep gas at elevated temperatures. The sweep gas may be any gas or gas mixture that is not reactive (i.e., inert) under calcination conditions, but is effective to remove moisture from the catalyst and restore the Cs-Ag promoter interaction. Such gas includes air, helium, hydrogen, oxygen, argon, carbon dioxide, nitrogen, and combinations thereof. Preferably, the sweep gas is air. Also preferably, the sweep gas has a moisture content of 1000 ppm or less.
The flow rate of sweep gas required in the process of the invention is a function of the amount of catalyst to be treated. This can be determined by routine experimentation by those skilled in the art. Generally, the sweep gas may have a gas hourly space velocity of about 10-10,000 hrxe2x88x921.
Preferably, the calcination is carried out at an elevated temperature between 150-350xc2x0 C., and more preferably, between 200-300xc2x0 C. It has been discovered that calcination in this temperature range can restore catalytic activity to levels between 85-100% of that for freshly prepared catalyst.
The length of time needed to restore catalyst performance is related to the temperature of calcination. For example, air calcination at 225xc2x0 C. requires 4-8 hours to give approximately 95-100% restoration of activity, while calcination at 250xc2x0 C. requires only 1-2 hours. Thus, lower calcination temperatures would require longer calcination periods, and vice versa.
The specific length of time required to restore the catalyst to its initial activity/selectivity also depends on the amount of catalyst deactivation. Generally, calcination for a period of 1-30 hours at the above temperatures would be sufficient. Preferably, the heat treatment is carried out for a period between 4-23 hours, and more preferably between 4-15 hours.
The restorative calcination step may be carried out in a batch, semi-continuous, or continuous mode of operation. For example, batch operation may comprise heating the catalyst at the requisite temperature in a tubular reactor vessel while introducing a stream of air into the reactor. After the activity and selectivity of the catalyst have been restored, the flowing air may be stopped and the reactive gases such as ethylene or butadiene and oxygen plus an optional diluent may be introduced into the reactor to initiate the desired reaction.
Thus, another aspect of my invention relates to the preparation of 3,4-epoxy-1-butene. This embodiment may be carried out by contacting 1,3-butadiene with oxygen in the presence of the heat-treated catalyst described above at conditions effective to produce the 3,4-epoxy-1-butene. The specifics for carrying out this reaction are well known in the art and are described in the patent literature such as U.S. Pat. Nos. 4,950,773; 5,117,012; and 5,618,954; the entire contents of which are hereby incorporated by reference.
As an optional step, the heat-treated catalyst may be intimately contacted with a gas stream comprising about 4 to 50% by volume of hydrogen at a temperature of about 170xc2x0 C. to 400xc2x0 C. and a gas hourly space velocity of about 10 to 10,000 hrxe2x88x921 to further improve or enhance the activity/selectivity of the catalyst. The balance of the gas stream can be any inert gas known in the art such as nitrogen, helium, argon, carbon dioxide, methane, or a mixture thereof. This H2 treatment step may be carried out in accordance with the teachings of U.S. Pat. No. 5,081,096; the entire content of which is hereby incorporated by reference. I have found that Cs-promoted catalysts given this H2 treatment after the heat treating step can activate more quickly, and possibly even to a higher level of catalytic activity, than heat-treated Cs-promoted catalysts not given the additional H2 treatment.
While the foregoing discussion has been focused on Cs-promoted, Ag catalysts, it is believed that all fresh Cs salt-promoted catalysts are moisture-sensitive and are prone to deactivation upon storage at ambient conditions. Thus, all such catalysts would benefit from storage in a substantially moisture-free environment and/or a calcination treatment immediately prior to being used to give optimum catalytic activity and selectivity greater than otherwise observed.